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This invention relates to light-emitting devices, or more specifically to a class of semiconductor light emitting diodes known as super-luminescent light emitting diodes, which are constructed as multilayered structures having vertical cavities that, on the minimum, comprise a substrate base, a light reflecting mirror structure, which is typically a quarterwave mirror stack assembly, a double-heterostructure (LED) xe2x80x9cLight Emitting Diodexe2x80x9d, and a window emitter-layer.
Side-emitting xe2x80x9cLight Emitting Diodesxe2x80x9d (LEDs) are well known semiconductor light emitting devices in which, electrical current (i.e., electrical pumping) is made to pass through a diode junction to produce light emissions within an active layer of semiconductor material, which is located within the p-n junction of the previously mentioned diode itself. At least one facet of a side-emitting LED device is coated with an anti-reflective material, which will cause light emissions to exit the coated facet. This is to be contrasted with a side-emitting light emitting diode laser, where stimulated emission of light is also made to occur within the light emitting diode""s junction. Stimulated emission occurs when the electrically pumped fundamental light already created within the light emitting diode""s double-heterojunction is made to optically stimulate the double-heterojunction""s active semiconductor layer or layers, which are also normally located between the side emitting diode""s two contra-opposed light reflecting crystal facets.
Wherein, repeated reflections of light are made to oscillate through the diode junction""s active semiconductor layer or layers, back and forth, between the diode""s previously mentioned contra-opposed light reflecting crystal facets, causing a coherent laser beam to emerge. The resulting laser beam usually has a very narrow spectral width (i.e., meaning monochromatic). Non-laser light emitting diodes that operate at a relatively higher power over other LEDs, while having a relatively broad spectral width are within a third category of devices called super-luminescent light-emitting diodes. There is a need for these devices, when they are used in fiber optic systems having a requirement for low Raleigh backscattering, such as in fiber optic gyroscopes or devices needing low modal noise. Commercially available super-luminescent light-emitting diodes typically emit light at powers as high as xe2x80x9c4xe2x80x9d to xe2x80x9c6xe2x80x9d mW (i.e., milliwatts).
However, when the power in these devices is increased above xe2x80x9c1xe2x80x9d to xe2x80x9c2xe2x80x9d mW, the frequency spectrum is substantially narrowed. Driving devices with contra-positioned edge-emitting facets to higher powers may eventually cause lasing, in spite of the presence of the anti-reflective coating on the previously mentioned facets, since even the best anti-reflective coating will reflect some proportion of the light impinging on it, and lasing will eventually occur if the power is increased to a high enough level. The lasing threshold for pulsed diode operation increases with decreased facet reflectivity. The only successful high-power anti-reflective coated super-luminescent diodes were made by dynamically monitoring the pulsed laser threshold during the coating process. For this reason, the anti-reflective coatings in super-luminescent light-emitting diodes have to be carefully controlled to permit operation at higher power levels. When a super-luminescent diode having one or both facets coated with an anti-reflective material is operated at a high enough current, the spectral content of the output light may still cover a desirably broad band of wavelengths.
However, above a certain power level the device operates more and more like a laser and its output spectrum is characterized by narrow modal lines spread over a relatively broad band. In this lasing mode of operation, the device is said to operate with a high degree of Fabry-Perot modulation, the characteristic laser-cavity modulation that is undesirable for applications like the fiber optic gyroscope. These applications require very low Raleigh back-scattering noise, which can only be obtained with a low coherence length and a wide spectral width. As the power of a side-emitting super-luminescent light emitting diode is increased and its spectral width is consequently decreased, the coherence length of light from the device is increased. The coherence length is another measure of the spectral purity of light, and is inversely proportional to spectral width. As the spectral width becomes narrower, the coherence length increases.
Moreover, if the edge-emitting device operates with a large degree of Fabry-Perot modulation and moves into a lasing mode, the coherence length is inversely proportional to the spectral width of the individual modal-lines within the intensity-wavelength characteristics of the device. Thus, the coherence length for the lasing mode of operation is several orders of magnitude larger than the coherence length for a super-luminescent diode. The requirement for a light-emitting device with low coherence length and relatively high power is simply not attainable with presently available super-luminescent diodes using antireflective coatings to suppress lasing. The cross-referenced U.S. Pat. No. 4,634,928 proposes one technique for the suppression of lasing in a light-emitting device. That approach employs means within the semiconductor structure for producing a non-uniform gain profile along the active layer of the device. The non-uniformity of the gain profile results in a broadening of the frequency spectrum of emitted light. As the power is increased, the spectral width increases even more, permitting the output of relatively high powers while maintaining a broad spectral width.
Some years ago, D. R. Scifres et al. reported in the IEEE Journal of Quantum Electronics, QE-14, 223 (1978), that he experimented with a different type of structure that showed promise as a super-luminescent diode. Conventionally, a side-emitting semiconductor laser is constructed to laze in a direction normal to the crystalline cleavage plane along which the facets are formed. These researchers constructed a laser at an angle inclined to the normal direction, such that light propagating at an internal angle of zero, i.e. parallel to the longitudinal direction of the laser, would impinge on the facets at a small angle to the perpendicular. The Scifres et al. cavity structure was of the xe2x80x9cgain-guidedxe2x80x9d type of optical cavity.
Moreover, all light-emitting semiconductors emit light from a diode junction to which power is supplied electrically from a contact stripe formed on the device. If a narrow electrical contact is employed to supply the current, lasing action is typically limited to a correspondingly narrow region, with the lateral waveguide boundary defined by the gain profile, i.e. with no intentional refractive index profile built into the structure. This process is generally referred to as gain guiding. The Scifres et al. side-emitting device was run in a pulsed mode and, although super-luminescence was observed, a large proportion of the output was due to lasing.
Moreover, there was an observed tendency at higher currents for the internal beam angle to move toward zero, which minimizes reflectivity losses at the facets and pushes the device more strongly into lasing operation. It will be appreciated from the foregoing that there is still a need for a super-luminescent diode with the characteristics of high power, large spectral width, and low Fabry-Perot modulation. Specifically, the requirement is for a device operable at powers more than xe2x80x9c10xe2x80x9d mW (i.e., milliwatts), a spectral half-width of at least xe2x80x9c50xe2x80x9d angstroms, and at most 10% Fabry-Perot modulation. The present invention meets or exceeds these requirements without difficulty. The present invention has a redefined Fabry-Perot modulation neutralizing vertical folded cavity design. Inspired by the present invention""s unique Fabry-Perot modulation neutralizing folded cavity and by the optical physics that occur therein, for future identification, the present invention has been named the xe2x80x9cFolded Cavity Light Emitting Diodexe2x80x9d (FCLED).
In addition, vertical cavity surface emitting diodes have employed xe2x80x9cAluminum-Gallium-Indium-Phosphidexe2x80x9d (AlGaInP) alloy technology for making xe2x80x9cLight Emitting Diodesxe2x80x9d (LEDs) of wavelengths ranging from about xe2x80x9c550xe2x80x9d to xe2x80x9c680xe2x80x9d nanometers by adjusting the aluminum to gallium ratio in the active region of the previously mentioned LEDs. Further, xe2x80x9cMetalorganic Vapor Phase Epitaxyxe2x80x9d (MOVPE) is used to grow efficient AlGaInP heterostructure devices. While, a conventional LED contains a double heterostructure of AlGaInP, which includes an n-type AlGaInP cladding-layer formed on an n-type substrate of xe2x80x9cGallium-Arsenidexe2x80x9d (GaAs), an active layer of AlGaInP formed on the n-type cladding-layer, and a p-type AlGaInP cladding layer formed on the active layer.
Furthermore, for efficient operation of the previously mentioned LED, injected current should be spread evenly in the lateral direction of the device, so that the current will cross the p-n junction of the double heterostructure of AlGaInP uniformly to generate light evenly. The p-type AlGaInP cladding-layer, which is grown by MOVPE process, is very difficult to dope with acceptors of a concentration higher than 1 E18 cm.sup.-3. Further, hole mobility (i.e., about 10 to 20 cm.sup.2 *v/sec) is low in p-type AlGaInP semiconductors. Due to these factors, the electrical resistivity of the p-type AlGaInP layer is comparatively high (i.e., about 0.3-0.6 .OMEGA.-cm normally), so that current spreading is severely restricted.
Consequently, current tends to concentrate, and is often referred to as the current crowding problem. One technique to solve the current crowding problem is disclosed by Fletcher et. al. in U.S. Pat. No. 5,008,718. The structure of the proposed LED is fabricated with a back electrical contact, a substrate of n-type GaAs, a double heterostructure of AlGaInP, a window-layer of p-type GaP, and a front electrical contact. The double heterostructure of AlGaInP mentioned above includes a bottom-cladding layer of n-type AlGaInP, an active layer of AlGaInP, and a top cladding layer of p-type AlGaInP. The window-layer should be selected from materials that have a low electrical resistivity so that current can spread out quickly, and have a band gap higher than that of the AlGaInP layers so that the window-layer is transparent to light emitted from the active layer of AlGaInP.
In addition, an LED for generating light in the spectrum from red to orange, AlGaAs material is selected to form the window-layer. The AlGaAs material has the advantage of having a lattice constant compatible with that of the underlying GaAs substrate. In an LED for generating light in the spectrum from yellow to green, GaAsP or GaP material is used to form the window-layer. It is a drawback of using the GaAsP or the GaP material that their lattice constants are not compatible with those of the AlGaInP layers and the GaAs substrate. This lattice-mismatch causes a high dislocation density that produces less than satisfactory optical performance. In Applied Physics Letter, vol 61 (1992), p. 1045, K. H. Huang et. al. discloses a similar structure having a thick layer of about 50.mu.m (i.e., or 500000 angstroms) in thickness. This structure provides a three times luminance efficiency than an LED without a window-layer, and two-times luminance efficiency than an LED with a window-layer of about 10 .mu.m in thickness.
Moreover, the fabrication of this structure unfavorably requires two different processes of xe2x80x9cMetalorganic Vapor Phase Epitaxyxe2x80x9d (MOVPE) for growing the double heterostructure of AlGaInP, and xe2x80x9cVapor Phase Epitaxyxe2x80x9d (VPE) for forming the thick window-layer of GaP; thereby, increasing manufacturing time and complexity. Another prior art super-luminescent LED design, which is disclosed in U.S. Pat. No. 5,048,035 is described as being fabricated with a current-blocking layer of AlGaInP on a portion of the double heterostructure, and a contact layer of GaAs located between the window-layer and the electrode. The current-blocking layer is arranged at a position where it is in alignment with the front electrode and thus current is spread out laterally by the current-blocking layer.
Moreover, two MOVPE processes are disadvantageously required in fabricating this structure, i.e. forming the heterostructure and the current-blocking layer by a first MOVPE, followed by a photolithography technique to define the area of the current-blocking layer, and forming the window-layer by a second MOVPE. Another prior art super-luminescent LED as disclosed in U.S. Pat. No. 5,359,209 is described as being fabricated with an additional p-type window-layer of GaAs located between the heterostructure and the p-type window-layer of GaP. Although the window-layer of GaAs has good conductivity with carrier concentration of 10.sup.19 cm.sup.-3 or more, the structure induces a light absorption phenomenon because the energy gap of GaAs is substantially less than that of AlGaInP in the active layer. In addition, another prior art super-luminescent LED design, as disclosed in U.S. Pat. No. 5,481,122 describes the previously mentioned window-layer of xe2x80x9cGallium-Phosphidexe2x80x9d (GaP) as being replaced by a p-type contact-layer and a conductive transparent oxide-layer. Moreover, xe2x80x9cIndium Tin Oxidexe2x80x9d (ITO) is preferably used for forming the previously mentioned conductive transparent oxide layer, which has a high transparency rate of about 90% in the range of visible light. Further, its electrical resistivity (i.e., about 3.times.10.sup.-4 .OMEGA.-cm) is about xe2x80x9c1000xe2x80x9d times smaller than that of p-type AlGaInP, and about xe2x80x9c100xe2x80x9d times smaller than that of p-type GaP. However, the optimal thickness of about xe2x80x9c1000xe2x80x9d to about xe2x80x9c50000xe2x80x9d angstroms does not provide a good condition for effectively emitting light, thereby confining the luminance efficiency of the LED.
In addition, prior art as illustrated in FIGS. 1, 2, 3, and 4 describes super-luminescent xe2x80x9cLight Emitting Diodesxe2x80x9d (LEDs) as having, first of all, a back electrical contact 13, which is provided to act as a back electrode. Typically, prior art, as illustrated in FIGS. 1, 2, 3, and 4 describes a back electrical contact 13 as being made from a material that provides an n-type electrode. However, it must also be appreciated that a p-type electrode 13 can be used instead of the previously mentioned n-type electrode 13 without departing from the scope of the claimed invention. A substrate 14 layer is then formed on the n-type electrode 13.
Moreover, prior art as illustrated in FIGS. 1, 2, 3, and 4 describes the previously mentioned substrate layer 14 as being constructed from a n-type xe2x80x9cGallium-Arsenidexe2x80x9d (GaAs) binary semiconductor material, which is conventionally formed to a thickness of about 350.mu.m onto the outermost surface of the previously mentioned n-type electrode layer 13, using a known growth method. A layer stacked double-heterojunction structure of xe2x80x9cAluminum-Gallium-Indium-Phosphidexe2x80x9d (AlGaInP) 17 is often referred to as active p-n junction layer, and is often formed on the substrate 14 of GaAs. Prior art as illustrated in FIGS. 1, 2, 3, and 4 shows that the previously mentioned multi-stacked structure 17 includes a bottom n-type cladding-layer of xe2x80x9cAluminum-Gallium-Indium-Phosphidexe2x80x9d (AlGaInP) 17A, an active-area layer of AlGaInP 17B (i.e., typically un-doped), and a top p-type cladding-layer of AlGaInP 17C. The thickness of the bottom cladding-layer 17A, the active-area layer 17B, and the top cladding-layer 17C is preferably about 1.0, 0.75, and 1.0.mu.m respectively.
Moreover, in one implementation of prior art the active-area layer 17B is formed using a conventional xe2x80x9cDouble Heterostructurexe2x80x9d (DH) technique. In another implementation of prior art an active-area layer 17B can also be formed using another yet conventional structure typically called a multiple quantum well (MQW). A MQW, owing to quantum effect, will decrease the proportion of xe2x80x9cAluminumxe2x80x9d (Al) located within the previously mentioned active layer 17B; effectively reducing the proportion of oxygen located therein. Consequently, the quality of the crystalline is improved, and the illuminance efficiency of the prior-art super-luminescent LED is increased accordingly. Further, carrier overflow phenomenon is also reduced, confining carriers inside the quantum wells, because the carrier concentration in each quantum well is reduced as the number of the quantum wells becomes large.
Moreover, there are generally two types of multiple quantum well structures that might be used by prior art:
(i) A step index separate confinement MQW, in which there are step-shaped confining layers (i.e., not shown) located between the cladding-layers 17A, 17C, and the MQW active-area layer 17B.
(ii) A graded index separate confinement MQW, which uses graded confining-layers within the cladding-layers 17A, 17C, and the MQW active-area layer 17B. However, for a shorter wavelength (i.e., about 575-nm to 560-nm) LED structure having a weaker quantum well confinement, the previously mentioned graded index separate confinement MQW will achieve a better illuminance efficiency since it contains a reduced density of states in the previously mentioned confining-layers.
Moreover, prior art as illustrated in FIGS. 1, 2, 3, and 4 also describes a p-type window-layer 18 with a thickness of about 10.mu.m as being formed on the top cladding-layer 17C. Where, the electrical resistivity of the previously mentioned window-layer 18 (i.e., about 0.05.OMEGA.-cm) is less than or equal to that of the top cladding-layer 17C. A transparent material, such as xe2x80x9cGallium-Phosphidexe2x80x9d (GaP), xe2x80x9cGallium-Arsenide-Phosphidexe2x80x9d (GaAsP), xe2x80x9cGallium-Indium-Phosphidexe2x80x9d (GaInP), or xe2x80x9cAluminum-Gallium-Arsenidexe2x80x9d (AlGaAs) is preferably used. This window-layer 18 is typically used to improve the illuminance efficiency of super-luminescent LEDs. For example, a conventional 590-nm LED without a window-layer usually has a brightness of fifteen xe2x80x9cMinicandelaxe2x80x9d (MCD). However, 30-mcds or more can be obtained by adding the previously mentioned window-layer 18 (FIGS. 1, 2, 3, and 4) onto the outermost surface of the previously mentioned top cladding-layer 17C. Prior art also shows that the window-layer 18 can also be formed as a compositional graded window-layer by gradually adjusting the composition of AlGaInP material within the window-layer""s structure.
In addition, prior art as illustrated in FIGS. 1, 2, 3, and 4 describes a staircase formation of the previously mentioned window-layer 18 (FIG. 3), where the proportion of Ga, In, or Al is changed stepwise along with the thickness of the window-layer 18 itself. Prior art, as illustrated in FIGS. 1, 2, 3, and 4 shows another similar situation, where the proportion of Ga, In, or Al is changed linearly. Similarly, prior art also describes a sub linear formation, and a super linear formation. Wherein, the staircase and the linear formation of the window-layer have the advantage of high controllability and stability, where the sub linear formation has the advantage of a thicker window-layer because of its low defect density, while the super linear formation can be used to increase illuminance efficiency by lowering resistivity of the previously mentioned window-layer 18. More generally, use of a compositional graded window-layer 18 makes the window-layer""s lattice-constant compatible with that of the underlying double-heterostructure diode layer 17.
In addition, prior art also describes the use of a p-type contact-layer 19 that is directly formed on the window-layer 18 using a GaAsP, GaP, GaInP, or GaAs semiconductor material. Prior art, also describes the carrier concentration within the previously mentioned contact-layer 19 as being greater than 5.times.10.sup.18 cm.sup.-3, and its thickness as being no greater than 500 angstroms, so that a good ohmic contact can be formed between the window-layer 18 and a conductive transparent oxide-layer 20. Where, the electrical resistivity of the conductive transparent oxide-layer 20 (i.e., about 3.times.10.sup.-4 .OMEGA.-cm) is smaller than that of the contact-layer (i.e., about 0.01 .OMEGA.-cm) and the window-layer 18 (i.e., about 0.05 .OMEGA.-cm).
Moreover, xe2x80x9cTin Oxidexe2x80x9d (TO), xe2x80x9cIndium Oxidexe2x80x9d (10), or xe2x80x9cIndium Tin Oxidexe2x80x9d (ITO) is preferably used to form the previously mentioned conductive transparent oxide-layer 20. The preferred thickness of the conductive transparent oxide-layer 20 is between xe2x80x9c1000xe2x80x9d to xe2x80x9c50000xe2x80x9d angstroms. Therefore, the transmittance of the conductive transparent oxide-layer 20 is excellent for LEDs in the wavelength range of from 550-nanometers (i.e., green) to 630-nanometers (i.e., red). The conductive transparent oxide-layer 20 does not absorb photons emitted from the active-region 17, and its electrical resistivity is only about 3.times.10.sup.-4 .OMEGA.-cm, preferably, so that the injected current may substantially spread out through the entire diode, thereby contributing to higher power output.
Moreover, the window-layer 18 is not thick enough to provide good spreading capability for overcoming the current crowding problem, therefore the window-layer 18 and the conductive transparent oxide-layer 20 work together to provide prior nit super-luminescent LEDs with high-brightness. Prior art as illustrated in FIGS. 1, 2, 3, and 4 shows that 50-mcd or more can be obtained, compared to 15-mcd for a conventional LED, without the window-layer 18 and the conductive transparent oxide-layer 20. Finally, a p-type electrical contact 21 is typically formed on a portion of the conductive transparent oxide-layer 20 to act as a front electrode. It is noted that each layer, except the previously mentioned conductive transparent oxide-layer 20 and the electrodes 13, 21 (FIGS. 1, 2, 3, and 4) can be grown using a xe2x80x9cMetalorganio Vapor Phase Epitaxyxe2x80x9d (MOVPE) method, thereby achieving high controllability of composition, carrier concentration, layer thickness, and simplifying manufacturing.
In addition, FIG. 3 shows a cross-sectional view that illustrates a typical super-luminescent LED as having a xe2x80x9cDistributed Bragg Reflectorxe2x80x9d (DBR) or layer stack assembly comprising layered sections 16. Materials like AlGaInP or AlGaAs are preferably used in forming DBR layers 16 (FIGS. 1, 2, and 3), which typically includes a mirror stack of more than xe2x80x9c20xe2x80x9d layers. The DBR layer 16 is primarily used to eliminate the absorption of the fundamental light produced by and emitted from the active-area layer 17B by the substrate-layer 14, thereby increasing extraction efficiency of the prior art super-luminescent LED. Prior art, describes the DBR 16 (FIGS. 1, 2, and 3) as being typically grown onto the top and outermost surface 15 of a substrate-layer 14.
Furthermore, to better understand the structural differences between the present FCLED invention and art super-luminescent LED prior art technology, a typical example of a prior art high-frequency xe2x80x9cSuper-Luminescent Light Emitting Diodexe2x80x9d (SLLED) design is described in detail below. Furthermore, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows a SLLED design that is slightly different than the SLLED design described previously. However, while this new SLLED design still uses the well-known process of recombination xe2x80x9celectron/holexe2x80x9d radiation (i.e., what is sometimes called xe2x80x9cradiative recombinationxe2x80x9d) to produce intra-cavity fundamental light it is only conductive at its diode active-region. The prior art, illustrated in FIGS. 5, 6, 7, 8, 9 and 10 to begin with shows a high-frequency version of a SLLED design that uses a metallic supporting substrate 22 (FIGS. 5, 6, 7, and 8) as both a base-reflecting mirror structure 22 and as a substrate-layer that is used for the subsequent growth of its various multilayered structures. This is where SLLEDs typically begin the process of epitaxially growing contiguous layers using a well-known epitaxial process like xe2x80x9cMolecular Beam Epitaxyxe2x80x9d (MBE) or xe2x80x9cMetal Organic Chemical Vapor Depositionxe2x80x9d (MOCVD) for material deposition.
Furthermore, a SLLED""s metallic supporting substrate 22, when made conductive, as an alternative embodiment, would serve as the SLLED(s) electrically negative electrode. The metallic supporting substrate 22 is comprised from a (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d alloy-mixture, which has between an xe2x80x9c8.0xe2x80x9d to a xe2x80x9c12.0xe2x80x9d percent material lattice-mismatch, or more specifically, a xe2x80x9c10.0xe2x80x9d percent material lattice-mismatch to the binary (GaN) xe2x80x9cGallium-Nitridexe2x80x9d semiconductor material deposited later. Nevertheless, despite a (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d lattice-mismatch it is still the preferred metallic alloy-mixture used for this kind of electron conducting metallic supporting substrate 22. In addition, the (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d metallic supporting substrate 22 (FIG. 7), if used as an alternative embodiment, would also need to exhibit a highly reflective property as well and, therefore should have a surface roughness of less than xe2x80x9c15xe2x80x9d atoms thick.
Furthermore, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows several thin layers of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d material, which are grown layer-upon-layer using MBE or MOCVD as the deposition process, until a buffer-layer 23 (FIG. 7) is built up epitaxially that has a thickness of only a few atoms. This buffer-layer 23 is used for facilitating the xe2x80x9cMolecular Beam Epitaxyxe2x80x9d (MBE) or xe2x80x9cMetal Organic Chemical Vapor Depositionxe2x80x9d (MOCVD) epitaxial growth of the many subsequent semiconductor layers that will entirely comprise the high-frequency SLLED device.
In addition, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that typically a high-frequency short-wavelength SLLED design would have a lower xe2x80x9cDistributed Bragg Reflectorxe2x80x9d (DBR) or quarterwave mirror stack assembly 24 grown, epitaxially, onto the top and outermost surface of the previously mentioned buffer-layer 23A, 23B, 23C, 23D (FIGS. 5, 6, 7, and 8) of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d material using any suitable epitaxial crystal growing method, such as MBE or MOCVD.
Moreover, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a high-frequency SLLED""s quarterwave mirror stack assembly 24 is made from a plurality of alternating layers comprising mirror pairs; or more precisely, as comprising a multitude of single pairs of alternating layers 24A, 24B (FIGS. 5, 6, 7, and 8), which are constructed from a pair of xe2x80x9cGallium-Nitridexe2x80x9d and xe2x80x9cAluminum-Gallium-Nitridexe2x80x9d (GaN/AlGaN) semiconductor materials that are used to complete a single mirror pair. A plurality of alternating layers, which include one or more layers of N-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d 24A, 24C, 24E, 24G, 24I (FIGS. 5, 6, 7, and 8), a high refractive semiconductor material, and N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d 24B, 24D, 24F, 24H, 24J (FIGS. 5, 6, and 7), a low refractive semiconductor material.
For example, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a layer 24A of N-doped (GaN) xe2x80x9cGallium-Nitride is epitaxially deposited onto the top and outermost surface of a SLLED""s last buffer-layer 23D (FIGS. 5, 6, 7, and 8), while a layer 24B (FIGS. 5, 6, 7, and 8) of N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d is subsequently and epitaxially deposited onto the top and outermost surface of the SLLED""s first N-doped (GaN) xe2x80x9cGallium-Nitride layer 24A, which form a SLLED""s first single mirror pair. If additional mirror-pairs are required, several more layers are used to make-up additional mirror-pairs, which are deposited, epitaxially, onto the existing layers of (GaN) xe2x80x9cGallium-Nitride and (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d materials 24A, 24B, 24C, 24D, 24E, 24F, 24H, 241 (FIGS. 5, 6, 7, and 8).
Moreover, to increase the reflectivity of a SLLED""s quarterwave mirror stack assembly 24 (FIGS. 5, 6, 7, and 8) to any required amount of reflectance, many additional mirror pairs may be required, and depending on the frequency of light being reflected, as many as several hundred pairs might be needed and used. However, it should be understood that the thickness and doping levels of each deposited layer used within a prior-art SLLED design must be precisely controlled. Any deviation from designed parameters, no matter how slight, would affect the performance of a prior-art SLLED device (i.e., frequency range, flux intensity). This greatly adds to the cost and complexity of manufacturing high frequency SLLED devices.
For example, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s emitter-layer 32 (FIGS. 5, 6, 7, and 8), because it was designed to emit super-luminescent high-frequency light having a wavelength of xe2x80x9c200xe2x80x9d nanometers, should also have a material thickness that is the same as the other alternating layers used to comprise the SLLED""s quarterwave mirror stack assembly 24, which in both cases have material thickness"" one-quarter of one wavelength of the optical radiation emitted by the SLLED device in question. Prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 also shows that typically all layers used to comprise a SLLED""s quarterwave mirror stack assembly 24 (FIGS. 5, 6, 7, and 8) shall, therefore have a material thickness of xe2x80x9c50xe2x80x9d nanometers.
Furthermore, the doping of a SLLED device is accomplished by the addition of various dopant materials (e.g., n-type electron donating dopants like Phosphorus and p-type electron accepting dopants like Boron) to epitaxial materials used in the MBE or MOCVD epitaxial deposition process of epitaxially deposited materials. Typically, a SLLED device will use many different dopant concentrations of specific dopant materials within the several different extrinsic semiconductor layers that make-up a SLLED""s various planar structures. For example, alternating layers of (GaN) xe2x80x9cGallium-Nitridexe2x80x9d 23A (FIGS. 5, 6, 7, and 8) and N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d 23B (FIGS. 5, 6, 7, and 8), which are used to facilitate construction of a SLLED""s quarterwave mirror stack assembly 24 (FIGS. 5, 6, 7, and 8) can be made n-type and, therefore conductive, when doped with either xe2x80x9cSeleniumxe2x80x9d or xe2x80x9cSiliconxe2x80x9d using a dopant concentration ranging from xe2x80x9c1 E15xe2x80x9d to xe2x80x9c1 E20xe2x80x9d cubic-centimeters with a preferred range from xe2x80x9c1 E17xe2x80x9d to xe2x80x9c1 E19xe2x80x9d cubic centimeters, while a nominal concentration range of doping would be from xe2x80x9c5 E17xe2x80x9d to xe2x80x9c5 E18xe2x80x9d cubic centimeters. The percentage of dopant composition used in a SLLED""s quarterwave mirror stack assembly 24 could be stated as (Al x Ga x N/GaN), where x represents a variable of xe2x80x9c0.05xe2x80x9d to xe2x80x9c0.96xe2x80x9d, while in a preferred embodiment x would represent an amount greater than xe2x80x9c0.8xe2x80x9d.
Therefore, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that once the plurality of alternating layers used in a SLLED""s first quarterwave mirror stack assembly 24 have been deposited onto the top and outermost surface of the SLLED""s buffer-layer of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d 23, then the SLLED""s first contact-layer 25 (FIGS. 5, 6, 7, and 8), which is comprised from a highly+n-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d binary semiconductor material can be epitaxially grown onto the top and outermost surface of the last alternating layer of the SLLED""s quarterwave mirror stack assembly 24 (FIGS. 5, 6, 7, and 8). A SLLED""s first contact-layer 25, while providing connectivity to the SLLED""s n-metal contact 27 (FIGS. 5, 6, 7, and 8), and to the SLLED""s n-metal contact-ring 26 (FIGS. 5, 6, 7, and 8), will also enhance the reliability of the SLLED""s design by preventing the migration of carrier-dislocations, and the like to the SLLED""s active-region 28 (FIGS. 5, 6, 7, and 8).
Furthermore, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that to prevent the overcrowding of the cladding-layers within a SLLED""s active-region 28, each cladding-layer is shown as being single and separate 28A, 28C (FIGS. 5, 6, 7, and 8). It should also be understood that each cladding-layer could be made using more than one layer, where each cladding-layer 28A, 28C is epitaxially deposited onto a previous cladding-layer, while each cladding-layer 28A, 28C is comprised from N-doped or P-doped (AlGaN) xe2x80x9cAluminum-Gallium-Nitridexe2x80x9d ternary semiconductor material, or any other suitable doped material available.
Furthermore, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s active-region 28 (FIGS. 5, 6, 7, and 8), which is shown as being represented by a single layer is as presented here comprised as a (SQW) xe2x80x9cSingle Quantum Wellxe2x80x9d epitaxially deposited onto the top and outermost surface of the SLLED""s first cladding-layer 28A (i.e., sometimes called a cladding-barrier). It should be understood, however, that a SLLED""s active-region 28 could also include one or more quantum-well cladding-layers and quantum-well layers, as is typical of MQW structures, or more particularly a first quantum-well cladding-layer and a second quantum-well cladding-layer, with a quantum-well layer positioned between them. Prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s active-area 28B is comprised as a SQW, which is constructed from a p-doped (InGaN) xe2x80x9cIndium-Gallium-Nitridexe2x80x9d extrinsic ternary semiconductor material, using MBE or MOCVD to epitaxially deposit the material onto the first cladding-layer 28A.
In addition, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s second contact-layer 29 (FIGS. 5, 6, 7, and 8), which is comprised from a highly+p-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d extrinsic binary material is epitaxially grown onto the top and outermost surface of the SLLED""s second cladding-layer 28C. A SLLED""s second contact-layer 29, while providing connectivity to the SLLED""s p-metal contact 31 (FIGS. 5, 6, 7, and 8) and to the SLLED""s p-metal contact-ring 30 (FIGS. 5, 6, 7, and 8) will also enhance the reliability of the SLLED""s design by preventing the migration of carrier-dislocations, and the like to the SLLED""s active-region 28.
In addition, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s emitter layer 32 (FIGS. 5, 6, 7, and 8), which is constructed from un-doped (ZnO) xe2x80x9cZinc-Oxidexe2x80x9d a high-refractive dielectric material is shown as being the last layer in the SLLED device to be deposited. In addition, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s p-metal contact 31 and the SLLED""s p-metal contact-ring 30 are formed onto the top and outermost surface of the SLLED""s second contact-layer 29 (FIGS. 5, 6, 7, and 8) by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys. Prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 also shows that a SLLED""s n-metal contact 27 (FIGS. 5, 6, 7, and 8) and the SLLED""s n-metal contact-ring 26 (FIGS. 5, 6, 7, and 8) are formed onto the top and outermost surface of the SLLED""s first contact-layer 25 (FIGS. 5, 6, 7, and 8) by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys.
Furthermore, it should also be understood that a chosen method of material deposition depends upon which material is selected for a SLLED""s electrical contacts 27, 31 (FIGS. 5, 6, 7, and 8). Therefore, specific methods of material disposition, disposing, and patterning onto the SLLED""s first and second contact-layers 25, 29, for any specific material, must be considered in the construction of the SLLED""s electrical contacts 27, 31 (FIGS. 5, 6, 7, and 8).
Moreover, prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows that a SLLED""s second contact-layer 29 (FIGS. 5, 6, 7, and 8), a SLLED""s second cladding-region 28C, a SLLED""s active-area 28B, and a SLLED""s first cladding-layer 28A (FIGS. 5, 6, 7, and 8) are all mesa-etched structures; moreover, defining the overall shape and structure of the SLLED""s lower layers, while their diameter dimensions remain substantially larger than the SLLED""s top deposited emitter layer 32 (FIGS. 5, 6, 7, and 8) and the emitter-layer""s support 29. As mesa etching is completed a SLLED""s p-metal contact 31 (FIGS. 5, 6, 7, and 8), and the SLLED""s p-metal contact-ring 30 (FIGS. 5, 6, 7, and 8) are deposited onto the top and outermost surface of the SLLED""s second contact-layer 29 leaving, therein the SLLED""s emitter-layer area open 32.
In addition, the deposition of a SLLED""s n-metal contact, as an alternative embodiment, can be deposited onto the top and outermost surface of the SLLED""s metallic supporting substrate 22 (FIGS. 5, 6, 7, and 8) of (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d alloy, which would allow the metallic supporting substrate 22 to function as an electrically negative contact-layer. Prior art as illustrated in FIGS. 5, 6, 7, 8, 9 and 10 shows a SLLED""s metallic supporting substrate-layer 22, when it is used in conjunction with a quarterwave mirror stack assembly 24 (FIGS. 5, 6, 7, and 8) constructed from mirror pairs of highly reflective (AlGaN/GaN) xe2x80x9cAluminum-Gallium-Nitride/Gallium-Nitridexe2x80x9d material, provide for approximately xe2x80x9c99.99xe2x80x9d percent of the SLLED""s total reflectivity. Furthermore, prior art as illustrated in FIGS. 8, 9, and 10 shows how high-frequency SLLED devices can be grouped together and configured as a linear array of high-frequency super-luminescent light emitting diodes.
In accordance with the present invention a super-luminescent folded cavity light emitting diode would have a cavity folding waveguide that comprises at least one total internally reflecting waveguide prism, which provides for the redirection of intra-cavity produced fundamental photonic radiation into and out of intra-cavity transverse propagation, a semiconductor double-heterojunction diode active-region that comprises an active-area, which provides for the production of fundamental photonic radiation, a photon collimating and photon focusing window emitter-layer, which is capable of collimating and focusing sufficient undiffused optical radiation into a propagation direction away from the present invention""s optically folded vertical cavity.
Objects and Advantages
Accordingly, besides the objects and advantages of the Super-Luminescent Folded Cavity Light Emitting Diode described in the above patent, several objects, and advantages of the present invention are:
(a) To provide a super-luminescent folded cavity light emitting diode that creates a high output of wide spectral light without using a quarterwave mirror stack assembly, but instead uses a cavity folding internal reflecting polyhedral prism waveguide, which is comprised from a single layer of dielectric or semiconductor material that is anisotropic or amorphous;
(b) To provide a super-luminescent folded cavity light emitting diode that is inexpensive to manufacture by eliminating the expensive epitaxial deposition of a bottom positioned quarterwave mirror stack assembly comprising a multitude of dielectric or semiconductor layers constructed using anisotropic or amorphous materials, and replacing it with a single dielectric polyhedral prism waveguide, which is constructed from a single inexpensive layer of sputter or epitaxially deposited material;
(c) To provide a super-luminescent folded cavity light emitting diode that uses two graded confinement cladding-layers to generate higher output emission;
(d) To provide a super-luminescent folded cavity light emitting diode that produces a more effective output gain using two graded confinement cladding-layers to lower the heat which is produced by electrical resistance between current conducting contact-layers and cladding-layers;
(e) To provide a super-luminescent folded cavity light emitting diode, which increases optical confinement with the addition of total internal reflecting cladding material to the surrounding vertical and outermost wall surfaces of the diode""s folded vertical cavity(s);
(f) To provide a super-luminescent folded cavity light emitting diode, which can be configured and controlled as a single light emitting diode device;
(g) To provide a super-luminescent folded cavity light emitting diode, which can be configured as a single diode-array that comprises a multitude of diodes, which are controlled as a single group of light emitting diode devices (i.e., sometimes called a diode-array) or controlled as a single group of independently controlled light emitting diodes;
(h) To provide a super-luminescent folded cavity light emitting diode or a super-luminescent folded cavity light emitting diode-array, which can be manufactured at the same time and from the same semiconductor substrate material used to construct the laser-array""s control-circuitry, all of which, would be contained within a single integrated semiconductor device;
(j) To provide a super-luminescent folded cavity light emitting diode that replaces a bottom quarterwave mirror stack assembly with a polyhedral prism waveguide which, if comprised of quartz or fused silica, can reflect one-hundred percent all frequencies of optical radiation entering a polyhedral prism waveguide""s top front-face flat horizontal surface using a process of total internal reflection;
(k) To provide a super-luminescent folded cavity light emitting diode, which can inexpensively construct its polyhedral prism waveguide using a well-known ion-milling process to slice out the waveguide""s prism facet(s);
(l) To provide a super-luminescent folded cavity light emitting diode that can deposit a dielectric material like fused-silica onto any construction material that might be used to construct any frequency producing semiconductor diode or combination thereof that could possibly be used to construct a super-luminescent folded cavity light emitting diode or a super-luminescent folded cavity light emitting diode-array;
(m) To provide a super-luminescent folded cavity light emitting diode, which uses an amorphous material like xe2x80x9cLithium-Fluoridexe2x80x9d (LiF) to create, for a vertical cavity or vertical cavities, an optical cladding material layer that is thermally dispersive and gives added support to a diode(s) polyhedral prism waveguide structure(s);
(n) To provide a super-luminescent folded cavity light emitting diode that increases its spectral-linewidth by applying anti-reflection coatings to its polyhedral prism waveguide(s);
(o) To provide a super-luminescent folded cavity light emitting diode that has eliminated the need to pre-deposit buffer layers of crystal growing materials like xe2x80x9cAluminum-Nitridexe2x80x9d onto a diode(s) substrate layer;
(p) To provide a super-luminescent folded cavity light emitting diode that produces an increase of nearly 7-mW to its output emission of fundamental light;
(q) To provide a super-luminescent folded cavity light emitting diode that can decrease its optical gain by coating its polyhedral prism waveguide""s facets with an anti-reflection material.
Further objects and advantages are to provide a super-luminescent folded cavity light emitting diode, where the selection of one semiconductor material over another, or the selection of one anisotropic or amorphous optical material over another that might be used in the construction of the super-luminescent folded cavity light emitting diode""s active-region, polyhedral prism waveguide, and emitter layers is not determined by any structural need or lattice compatibility, but by an application""s need for specific frequency(s). The materials used in the construction of the present invention, as presented here, are only, one, yet preferred, example of a group of several frequency-specific materials that might be used to construct the present invention""s frequency-transcendent multilayered structure. The advantage is that the novel features and the un-obvious properties that lie behind a super-luminescent folded cavity light emitting diode""s cavity folding structure, because they can exist, and occur, using any frequency-specific semiconductor or optical material, shows that the various structures that comprise the super-luminescent folded cavity light emitting diode(s) have a sufficient novelty and a non-obviousness independent of any one particular semiconductor or optical material that might or could be used in the super-luminescent folded cavity light emitting diode(s) construction. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.